The numbers I refer to are the bulb type numbers that most places seem to use. Halfords sell them under these designations too. Here are what the bulbs look like:

Type 509T (twist) bulb, as used in the Td5 speedo head. 12V 1.2W

Type 286 wedge bulb, as used in fuel/temp gauge and heater control illumination. 12V 1.2W

Fitting the Bulbs

For the heater illumination, fuel gauge and temperature gauge, fiting the bulbs is as easy as pushing them into the bulb holder. These then push into the gauges/binnacle where appropriate. Fitting new bulbs to the Td5 speedo head is slightly more involved though, and requires a pair of tweezers:

Remove the harness plugs from the back of speedo head:

Bulb in it's housing. Notice the flats on the bulb which you will grip with the tweezers

Insert the tweezers, and grip the flats of the bulb. Twist 90° anti-clockwise and the bulb should come free:

Twist anti-clockwise to remove bulb

Pick up the new bulb as shown below:

How the bulb should be held for removal/insertion

Carefully place the bulb into the slot. Take note of the metal wings on the bulb, these need to fall through the key slots visible in the empty hole. Once the bulb is fully home, twist clockwise 90° to lock it in place.